To unravel how the astonishing diversity of neurons is derived through birth order/timing-dependent cell fate specification, we propose to use sophisticated genetic tools in a relatively simple organism, the fruit fly Drosophila, to study the mechanisms of neuronal temporal identity. From a genetic mosaic screen, we have identified the Drosophila Chinmo BTB-zinc finger protein, and in particular its gradients through neurogenesis, as a novel mechanism conferring neuronal temporal identity. We have further demonstrated that the establishment of the Chinmo gradient involves the differential translation of chinmo messages carrying a 1.7kb-long 5'UTR. In this proposal, we will (1) examine the generality of Chinmo function in specifying neuronal temporal identity, (2) investigate the chinmo 5'UTR-dependent translational control for tracing back the origin for Chinmo-governed neuronal temporal cell fate specifications, and (3) screen for additional genes that may act within the Chinmo pathway or operate independently to control neuronal temporal identity. This study, including thorough analysis of Chinmo function and systematical identification of temporal identity genes, promises to provide major insights into the mechanisms governing birth order/timing-dependent neuronal diversification, a crucial developmental process in the brain. PUBLIC HEALTH RELEVANCE: The brain is composed of a monumental number of different types of neurons. These neurons interact with each other forming the complex brain circuits that underlie behavior and the coordination of the vital functions of the body. Congenital and developmental disorders that interfere with the formation of neuronal diversity have drastic consequences for brain function. Further, knowledge about the stem cells that in the brain give rise to this diversity may provide powerful clinical approaches for interventions during degenerative disorders or after injury. The goal of this project is to understand how brain stem cells give rise to neuronal diversity according to the order in which neurons are born (temporal identity). To address this question we are using a powerful model system, the fruit fly Drosophila, in which the genes controlling temporal identity can be isolated in an efficient manner. Our studies in this system have isolated a gene, called chinmo (chronologically inappropriate morphogenesis) with central roles in this process. In this project we will elucidate the mechanisms of Chinmo function by (a) determining if it is a universal temporal identity gene, or whether there are other genes that also can fulfill this function, (b) determining the mechanisms by which Chinmo functions, and (c) searching for other cell identity genes that may function with Chinmo, or in a Chinmo-independent fashion to confer temporal identity. We expect that the studies proposed will have a fundamental impact in our understanding of how neuronal diversity is generated, how this neuronal diversity is encoded in stem cells, and how these stem cells can be manipulated in order to generate particular neuron populations.